A conventional method for checking an exhaust gas cleaning plant is, for example, described in DE 199 05 192 A1 and is an example of an exhaust gas catalytic converter monitoring method that uses the catalytic converter that can store oxygen for the checking process. In this way, the fact that this catalytic converter that can store oxygen, correlates with the hydrocarbon conversion characteristic of the catalytic converter is utilized. If a catalytic converter has good hydrocarbon conversion capabilities, the lambda fluctuations of the air/fuel ratio by means of which the internal-combustion engine is operated, smooth out oxygen fluctuations called up by the lambda fluctuations of the catalytic converter. On the other hand, if a catalytic converter has no or only reduced conversion characteristics because of aging, contamination, leaded fuel or combustion malfunctions, a control oscillation upstream of the catalytic converter penetrates the catalytic converter downstream.
Therefore, on the one hand, with the generic method the air/fuel ratio is set by means of a corresponding intervention in the case of a lambda control device of the internal-combustion engine with an oscillation around lambda=1 that brings about an oxygen fluctuation in the exhaust gas that exceeds the normal operating ratios. At the same time, the NOx concentration is detected downstream of the catalytic converter to be checked and a suitable diagnostic value that is a measure for the aging condition of the catalytic converter is calculated. As an alternative to this method according to DE 199 05 192 A1, the signal of a linear lambda probe can also be evaluated downstream of the catalytic converter in order to measure the oscillation of the air/fuel ratio and with that the catalytic converter that can store oxygen and the aging condition of the catalytic converter.
However, with increasing importance of exhaust gas after-treatment in internal-combustion engines, it is not only necessary to be able to measure the aging condition of the catalytic converter, but also to detect cases in which the monolith of the catalytic converter is destroyed mechanically (so-called “empty catalytic converters”). It has been proven that such empty catalytic converters were not detected from time to time by means of conventional methods.